In portable equipment, such as mobile telephones, prismatic shaped batteries are generally preferred to rod shaped ones in particular to reduce the overall size of the equipment. This portable equipment, which must by definition be light, generally incorporates electronic circuits and components which are fragile and often demanding from the point of view of power consumption. It is thus desirable for the prismatic batteries used in such equipment to have high power density while being light and of the lowest manufacturing cost possible, for them to have sufficient longevity, in particular in the case of storage batteries, and for them to have during use all necessary guarantees from the safety point of view, as regards both the user and the contiguous electronic circuits.
An object of the present invention is to provide a prismatic battery which has a high volumic power density, is lighter, a lower manufacturing cost than the prismatic batteries of the prior art as a result of a new concept of the jelly roll type of winding and which also allows safety of use to be increased.
The prismatic batteries of the prior art, schematically shown in FIGS. 1 and 1A are formed by a metal container 1 of parallelepiped shape, within which is placed a non circular spiral winding 2, of the jelly roll type, which has then been impregnated by an electrolyte. It will be noted that, in order to obtain the highest possible power density, i.e. to be able to introduce into the container the greatest possible quantity of active materials, it is imperative that the electrolyte is added after winding 2 is put in place, because of the swelling phenomena. This winding, which is obtained from one composite strip including a layer of active material forming anode 2a, a porous separator 2b, a layer of active material forming cathode 2c and a second porous separator 2b, does not allow precise positioning of the electric connection zones of anode 2a and cathode 2c to be obtained. It is therefore necessary to provide intermediate connection means 5, 6 between said connection zones and external contacts terminals 7, 8 situated for example on a cover 4 hermetically sealed to container 1. These intermediate connection means 5, 6 have to be welded to the inside of the container, which means that it is difficult to control the quality thereof. Each additional weld represents a risk of malfunction, either because of mechanical shocks during use, or because of mechanical stress which will be exerted after assembly, in particular during charging/discharging cycles for a storage battery. This jelly roll is generally held compressed for example by means of two corrugated elastic plates 3a, 3b, arranged between the large walls of container 1 and said winding 2. These elastic plates also allow small variations in volume of winding 2 during charging/discharging cycles to be absorbed. This type of prismatic battery, in particular in the case of a storage battery, generally also includes a safety vent schematically shown at 9. This safety vent 9 is in fact necessary given the chemical reactions which take place and which can cause an increase in pressure and/or temperature.
The jelly roll has advantages as regards the current density which is theoretically available, but requires a certain number of precautions to be taken, both as regards the manufacture thereof and the installation thereof in the metal container. Indeed, to have a maximum current density, it is important for the winding to have good cohesion at every point, i.e. for all the alternate layers of active materials and separators to be perfectly in contact with each other. This result is easily obtained for the rectilinear portion of the winding as a result of the compression means, for example by means of elastic plates 3a, 3b. For the curved portions at each end, the cohesion essentially depends upon the manufacturing method. In the current state of the art, a jelly roll is obtained first by effecting a winding on a cylindrical core which is then removed prior to exerting compression to give it an ellipsoidal shape suited to the opening of the container. In order to reduce dislocations observed in the curved portion of the jelly roll, U.S. Pat. No. 5,658,683 proposes using a substantially rhombic shaped core. In all cases, the winding obtained has to be manipulated with precaution to be introduced into the container at the same time as the elastic plates. In series manufacturing it is thus necessary to have improved automated equipment, which will have an influence on the unit price of the final product.